(1) Field of the invention
The invention relates to an arrangement for simultaneous two-way transmission of data signals with a symbol frequency 1/T over circuits having a given bandwidth. Simultaneous two-way transmission in one and the same frequency band usually requires two separate one-way channels for which in the public telephone network use can be made of the four-wire circuits between junction exchanges. In most telephone networks, however, the circuits between subscriber and junction exchange are two-wire circuits, the local circuit between subscriber and terminal exchange being constituted by a nonloaded cable and the circuit between terminal exchange and juction exchange usually being constituted by a coil-loaded cable. The use of coil-loaded cables means that the available bandwidth of these two-wire circuits is restricted to approximately 3400 Hz. To enable simultaneous two-way transmission over such two-wire circuits both ends of these circuits comprise arrangements in which the one-way transmit channel and the one-way receive channel are connected to the two-wire circuit by means of a hybrid coupler (fork-circuit). These hybrid couplers are terminated by a balancing network for matching the cable impedance. In view of the varying length and types of cable the cable impedance is usually not precisely known so that the balancing network used in practice is a compromise network. This results in a direct leakage from transmit to receive terminal of the hybrid coupler. In addition, impedance discontinuities of the two-wire circuit result in signal reflections. Both effects result in echoes of the signals supplied at the transmit terminal of the hybrid coupler occuring at the receive terminal of this coupler. In voice transmission, the compromise balancing network is nearly always adequate to keep the level of the echoes within acceptable limits so that these echoes are not annoying provided their transit times do not become too long. In the case of long transit times the disturbing influence of the echoes can be considerably reduced by using an echo canceler in which approximated echo signals are produced from the signals at the transmit terminal of the hybrid coupler by means of an adaptive filter having adjustable filter coefficients, which approximated echo signals are subtracted from the signals at the receive terminal of this hybrid coupler to obtain residual signals which are substantially free from echoes. Adjusting the filter coefficients is usually based on a criterion for minimizing the mean-square error, the optimum adjustment being obtained in an iterative manner by means of a gradient algorithm.
The invention is the result of investigations into the possibilities of simultaneous two-way transmission of binary data signals at a rate of 6400 bits/s over two-wire circuits as they are found in the public telephone network if this network would be used as transmission facility for a public data network. However, the invention is not limited thereto as the same principles may be utilized for data signals having multi-level data symbols, for other data symbol rates and for other two-wire circuits of a comparable nature. Although the simultaneous two-way transmission of binary data signals at a rate of 6400 bits/s will be frequently mentioned hereinafter this should not be interpreted as a limitation of the range of utilization of the principles according to the invention.
(2) Description of the prior art
Reference (D. 1) discloses an arrangement for simultaneous two-way transmission of data signals with a given symbol frequency over two-wire circuits in the public telephone network. This arrangement is provided with a one-way transmit channel, a one-way receive channel, a coupler for interconnecting said channels and the two-wire circuit and an echo canceler comprising an adaptive filter with adjustable filter coefficients for producing from signals in the transmit channel an approximated echo signal which is differentially combined with signals in the receive channel for forming a residual signal, said filter coefficients being adjusted under the control of the residual signal and the signals in the transmit channel for minimizing a prescribed function of the residual signal.
This prior art arrangement does not make use of the character of the data signals and the echo canceler used therein is fully independent of the modulation techniques used for the actual data transmission over the two-wire circuit. As regards implementation in analog or digital techniques and as regards operation this echo canceler therefore does not differ from the prior art echo cancelers which are used for voice transmission in the telephone network and which are, for example, described in references (D.2) and (D. 3).
The upper limit for transmission over coil-loaded telephone cables (approximately 3500 Hz) may give rise to the thought that in the case of binary data signals (2-level data symbols) baseband transmission might be used for data rates up to approximately 6400 bits/s. The binary character of these data signals may then be utilized for realizing an attractive digital echo canceler in which the required multipliers are of a very simple implementation. However, both the two-wire circuits considered and the hybrid couplers usually employed therein comprise a number of transformers so that in case of baseband transmission the low frequencies in the spectrum of the binary data signals are suppressed. It is true that these low-frequency deficiencies as such can be corrected by applying quantized feedback in the receiver but in practice this technique does not offer a practical remedy because the frequency characteristic of the low-frequency suppression is not exactly known in view of the widely divergent numbers and types of transformers in the transmission path which increases the probability of cumulative errors in an inadmissible manner.
In the base of binary data signals it has already been suggested to avoid the problems caused by the transformers by utilizing one of the modulation methods which are known as "biphase modulation" and "delay modulation". The use of these two modulation methods results in binary (2-level) passband signals with little energy at the low frequencies and with many transitions between the two levels. Consequently, it is possible to use the simple digital echo canceler mentioned above, to permit transformers in the trasmission path and to recover the symbol frequency for regeneration in a rapid and simple manner. The great drawback of both modulation methods is the relatively large bandwidth the passband signal requires for the transmission of binary data signals at given data rates, or, put differently, the low relative data rate (date rate per Hz bandwidth) of the passband signal. As a consequence, it is not possible to transmit, by means of these modulation methods, binary data signals having a data rate of 6400 bits/s over the two-wire circuits in the public telephone network without removing the loading coils in the coil-loaded cable sections.